1. Field of the Invention
The present invention relates to a carcass ply producing apparatus for producing a carcass ply layer of a tire, a carcass ply producing method using this apparatus, and a pneumatic tire incorporating the carcass ply layer produced by this apparatus.
2. Description of the Related Art
Currently, a radial tire which is mainstream of a pneumatic tire generally comprises a carcass layer in which a portion between a pair of annular beads is reinforced and a plurality of cords are arranged in a radial direction of the tire (a meridional direction), and a plurality of belt layers in which the carcass layer located below a tread is reinforced by hoop effect, and the cords include a plurality of belt layers which are inclined in a circumferential direction of the tire. Rigidity of the carcass layer is large in the radial direction of the tire, but relatively small in the circumferential direction while the rigidity of the belt layer is large in the circumferential direction of the tire, but relatively small in the radial direction.
Therefore, a local stress can be generated in a belt end in the vicinity of a boundary between the carcass layer and the belt layer, which lowers durability of the tire. When the vehicle corners, it is known that a reaction force is abruptly reduced if the vehicle reaches a slip limit, and it is not possible to drive the vehicle in some cases.
On the other hand, in the case of bias tires, which existed prior to the manufacture of radial tires, rigidity of the tread is insufficient and has no cornering reaction force and, thus, motion is inferior. Further, it is known that shearing distortion is generated in the tire side wall due to flexing wherein the cords intersect each other, and that portion is prone to become fatigued and destroyed.
As a carcass ply producing apparatus for forming the carcass layer, there is a sticking apparatus of a carcass cord disclosed in Japanese Patent Application Laid-open No. 2000-52448 (prior art 1). This is an apparatus in which a core which is formed into a doughnut-like shape as a whole is driven in the circumferential direction, and carcass cord (ply cord) sent from a string-guide is extended in the meridional direction of the core and stuck on an outer peripheral surface.
This apparatus includes a swinging sending-out mechanism which reciprocates the string-guide provided on a tip end of the arm in the meridional direction of the core along the outer peripheral surface.
Japanese Patent Application Laid-open No. H.8-207509 (prior art 2) discloses a producing method of a pneumatic tire. According to this method, a rubber tape continuously reciprocates in its widthwise direction in a zigzag manner on a forming drum, bent at a widthwise end of the forming drum and wound around the forming drum, thereby forming a cylindrical body (carcass ply).
Further, Japanese Patent Application Laid-open No. S61-263805 (prior art 3) discloses a method in which a carcass ply comprising a plurality of cords arranged straightly is stuck on a forming drum having a plurality of cylindrical drums and then, the cylindrical drums are inclined through a predetermined angle, thereby partially changing the cord angle of the carcass ply. Outer peripheries of opposite side drums are pushed by the bands. This prior art 3 discloses a pneumatic tire in which a cord of a carcass layer located below a tread is largely inclined from a meridional direction, two carcass layers in which cords located on a tire side are radially arranged are laminated such that the inclined cords intersect with each other, and a reinforcing layer in which cords are arranged in its circumferential direction is provided on the outer peripheral surface of the layer.
In the case of the apparatus disclosed in the prior art 1, however, the string-guide only reciprocates along the meridional direction of the core. Therefore, when the ply cord is supplied in the meridional direction, a disposition angle of the ply cord can not changed halfway.
In the case of the carcass ply produced by the prior art 2, although the disposition angle of the ply cord is inclined, the disposition angle can not be changed halfway in the widthwise direction.
Whereas, in the case of the prior art 3, the disposition angle of the ply cord can be changed halfway in the widthwise direction. However, since the outer peripheries of the opposite side drums are pushed by the bands, there is a drawback that a curvature of the bent portion whose cord angle is changed is prone to become smaller. As disclosed in the prior art 3, even according to a method to apply tension at the time of vulcanization formation to soften the curve, if flattening becomes smaller to some degree, since the angle variation from the side to the tread is abrupt, it is difficult to soften the curvature of the bent portion. Therefore, a predetermined cord angle can not be realized using the prior art 3.
If the radius of curvature of the bent portion becomes small, there is a problem that a great shearing force is generated between the layers of the upper layer bent portion and the lower layer bent portion of the carcass layer of the double layered structure and thus, the bent portion is prone to be fatigued and may be destroyed. Further, since the bent portion is discontinuously changed in cord angle, there are problems that predictability of the slip limit at the time of cornering is low and improvement in safety is small like the radial tire.
In the prior art 3, since the outer peripheries of the opposite side drums are pushed by the band, there is a problem that surplus steps are required, which complicates the producing step of the carcass ply. There is also a problem that a region of the drum pushed by the band can not be changed in cord angle.
The present assignee newly developed a producing method of a ply for a carcass for forming a carcass layer in which the inclination angle of cord is changed in ever portion of the tire (this is unknown before the priority data of the present application). According to this method, even if flattening is small in some degree, carcass layer having small variation in cord angle can be formed. However, the following new problem is caused. That is, if the flattening is small, a width of the tread is relatively wide and thus, if the rigidity of the reinforcing layer in its circumferential direction provided on an outer periphery of the carcass layer is not increased, it is difficult to keep a predetermined shape of the tread of the tire, which lowers the running performance.